Balanced detector for altimeters



Oct. 29, 1946. w. R. MERCER Erm.

BALANGED DETECTOR FOR ALTIMETERS Filed June 9, 1944 4f- 49 zu WILLIAM e 8, BIEN INVENTORS R. MERCER 51. Emu:-

Patented st. 29, 1945 ZAiMZZ BALANCE!) DETECTOR FOR ALTIMETERS William R. Mercer, Hightstown, and Ben R.. Cole, Princeton, N. J., assignors to Radio Corporation of America, a corporation of Delaware application .rune 9, i944., serial No. 539,528

8 Claims.

Our invention relates to balanced detectors for altimeters or other distance measuring systems of the frequency modulated or FM type.

in application Serial No. 445,720, led June 4, 1942, in the name of Royden C. Sanders, Jr., entitled Frequency modulated altimeter or distance indicator, there is described a balanced beat frequency detector comprising diodes and a tuned line in which undesired amplitude modulation in an FM altimeter or radar system is balanced out. The present invention is an improvement on balanced detectors of this type.

An object of the present invention is to provide an improved detector of the balancedY type.

A further object of the invention is to provide an improved balanced detector for use in FM distance measuring systems such as altimeters.

A further object of the invention is to provide a detector of the balanced type that can be ad- J'usted to an exact balance with a minimum of difculty.

A still further object of the invention is to provide an improved balanced detector for centimeter Wave signals.

According to one preferred embodiment of the invention the detector comprises a tuned coaxial line and a pair of diodes connected in parallel relation across one end thereof by means of capacitors. The signal from the receiving antenna (which is the signal from a reflecting surface or object) may be applied to the coaxial line whereby this signal is fed to the diodes in parallel relation. The signal direct from the transmitter, which functions as a heterodyning signal, is fed to the diodes in push-pull relation as a result of the following arrangement:

The inner conductor of the coaxial line is split or bifurcated at the end adjacent to the diodes to form a loop or two-wire line which is capacity terminated to function as a resonant line. The heterodyning signal is applied to this split-line loop through a coupling loop positioned adjacent thereto. Since the two diodes are connected to thevinner coaxial line conductor at opposite sides of the split-line portion, the result is that the heterodyning signal is applied to them in pushpull relation. The resulting beat frequency signal, which may be either audio or intermediate frequency, is taken from the diodes through a pair of conductors that pass axially through the inner conductor whereby they are properly shielded from the radio frequency signals.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure 1 is a sectional view of a balanced detector embodying the invention,

Figure 2 is a view, partly in section, taken on the line II-II 0f Fig. l, f

Figure 3 is a sectional view taken on the line III-III of Fig. 1,

Figure 4 is a schematic diagram showing the electrical circuit of the detector illustrated in Figs. 1, 2 and 3,

Figure 5 is a simplified circuit diagram showing the audio frequency portion of the circuit i1- lustrated in Fig. 4,

Figure 6 is a View taken on the line VI-VI of Fig. 2 showing a double loop for coupling to the split-line loop, and

Figure 7 is a sectional view illustrating a modied structure for the detector shown in Fig. 1.

In the several figures, similar parts are indicated by similar reference characters.

Referring to Figs. 1, 2 and 3, the parallel feed portion of the detector comprises a tuned coaxial line consisting of an outer tubular conductor I0 and an inner conductor I I. The inner conductor II is a solid rod that has two longitudinal holes I2 and I3 therein through which pass a pair of conductors I4 and I6, respectively.

An annular spacing and supporting member Il is provided at the lower end of the coaxial line at which point it short circuits the line. The coaxial line is tuned by a slidable short-circuiting ring I8 that carries flexible contact fingers I9. The ring I8 may be adjusted by means of a pair of rods 2I and 22 that are slidable in the spacer ring Il.

Signal from the receiving antenna may be applied to the coaxial line l0. II through a pair of coaxial conductors 23, 24 as indicated in Fig. 2.

The upper end of the outer conductor l0 is joined to a transverse tubular section 26 into which the upper end of the inner conductor II extends. A pair of diodes 2l' and 28 of the lighthouse type are supported in opposite ends of the section 26 with their anodes I5 and 20 opposite the inner conductor II. The anodes I5 and 2c are conductively connected by means of clips 3| and 32 to the conductors Ill and I6, respectively, these conductors being brought outside the inner conductor II through holes in the sides of the conductor I I which join the longitudinal holes I2 and I3.

For radio frequency signals, the anodes I5 and 20 of the diodes are connected capacitively U through mica spacers 33 and 34 to opposite sides of the inner conductor I l.

Referring now to the push-pull portion of the circuit, the upper end of the inner conductor Il is split to form a two-wire line or loop 31 that is capacitively terminated, the di-electric for the capacity termination being a piece of. mica 36 in the example illustrated. Also, a loading resistor 35 is connected across the split end of conductor H to reduce the Q of the push-pull circuit con-- siderably below the Q of the parallel feed circuit. The loop circuit 31 is the portion of the inner conductor l I that surrounds the rectangular opening or split 31.

The heterodyning signal (the signal direct from the transmitter) is fed to the loop 31 through a loop 4I (Figs. 2 and 4) coupled thereto. In the example shown, the loop lll is a double loop for improving balance which is described and claimed in application Serial No. 538,052, led May 30, 1944, in the name of Daniel Blitz, and entitled Balanced detector for altimeters. If preferred, a single loop may be used to couple to the splitconductor loop 31. The flow of heterodyning current around the loop 31 is indicated in Fig. 4 by the arrows.

The upper end of the split-conductor loop 31 is balanced with respect to the transverse tubular member '26 (which, preferably, is grounded) by means of a capacity plate 42 which is eccentrically attached to a rotatable supporting rod 43. Thus, the anodes I and 2l) at a given instant are above and below ground potential, respectively, by a like amount. It will be apparent that this is the desired push-pull application of the heterodyning signal to the diodes 21 and 23.

The diodes 21 and 28 are supported in tubular members l5 and L31, respectively, by means of supporting rings 48v and 49. The tubular members f2.6 and l1 are fastened to the tubular member 26 in coaxial relation therewith by means of threaded clamping rings 5| and 52, respectively. The supporting rings 48 and 49 have flexiblei-lnger contact rings 53 and 54, respectively, mounted thereon. The contact rings 53 and 54- are insulated from their supporting rings 48 and 59 by means of mica washers 5E and 51 as illustrated.

The fingers of the contact rings 53 and 54 are in contact with the diode metal wall sections which are connected to the diode cathodes 5B and 59 (Figs. 4 and 5). Thus the cathodes 58 and 59 are connected at radio frequencies to the tubular section 26 through capacitors 5l and E52 (Fig. 4) which have the washers 5B and 51 (Fig. l) as the dielectric.

Before discussing in more detail the electrical circuit of the balanced detector, the double coupling loop 4l will be described with particular reference to Figs. 2 and 6. The two loops 4I are supported by a block 63 of insulating material and, as shown in Fig. 2, are positioned above the split conductor section 31. The loops IH are covered by a rectangular shielding can S5 that is attached to the tubular section 26.

As shown in Figs. 4 and 6, one of the loops 4| is connected through a resistor 66 to one side of the input line 61; the other end of this loop is connected to the shielding can 65. Likewise, the other loop lll is connected through a resistor 68 to the other side of the line 61; and the other end of this loop is connected to the shielding can 65. The line 61 preferably is of a type having a socket (not shown).

considerable loss whereby proper termination of the line is less critical.

The transmitter end of the line E1 is grounded at a point e9 for insuring balanced excitation of the line 61 from the transmitter and hence balanced excitation of the detector. However, grounding the transmitter end of the line B1 does not always eliminate all signal due to unbalance excitation for the reason that the grounded point t9 is not always exact as to location. It is because of this that the double loop il is employed. It will be apparent that the coupling to the detector loop 31 from any part of the double loop lll will include both sides of the line 61 and any current dierence will be averaged out.

Figs. 4 and 5 are schematic diagrams which show more clearly the electrical circuit of the balanced detector, Fig. 5 being a diagram of the audio frequency portionof the circuit only. The

radio frequency portion of the circuit, with its push-pull and parallel modes of operation, has been fully described. The audio or beat frequency portion of the circuit may include two transformer primary windings 1l and 12 which receive signal from the diodes 21 and 28, respectively, and which are connected in opposing relation. A secondary coil 13 supplies the audio signal to a frequency counter or other utilization circuit (not shown).

One end of the primary 11 is connected to the anode l5 through a resistor 1li which together with the capacity indicated at 1S acts as a radio frequency filter. The other end of the primary 1i is connected through a conductor 15 to the cathode 59 which is held at ground potential by a large electrolytic capacitor 1Q. Also, one end of the primary 12 is connected to the anode 2! through a resistor 11 which together with the capacity indicated at 13 acts as a radio frequency lter. The other end of the primary 12 is connected through a conductor .Eli and a loading and self-biasing resistor 82 to the cathode 58 which is grounded through a cathode pin and The resistor 82 is bypassed for` the beat frequency by a lter capacitor 83.

It will be noted that the diodes 21 and 28 are in series for direct current but are in parallel for the beat frequency signal. The D.-C. voltage drop across the resistor 82 opposes current ow due to contact potential at the diodes.

It will be understood that any signal due to amplitude modulation at the detector will be in phase at the anodes l5 and 2in and therefore will be balanced out in the transformer 1l, 12, 13. The beat frequency signal, on the other hand, will be degrees out of phase at the anodes H3 and 2l) whereby the beat frequency outputs of the two diodes will add and appear in the secondary 13.

Fig. '7 illustrates a different method of coupling from the transmitter to the two-wire line or loop 31. In this case the coupling is made by a pair of conductors 86 and 1 that extend through two additional longitudinal holes E33 and 82 in the inner conductor Il. The conductors 55 and 81 have resistors 9| and 92, respectively, in series therewith for terminating the line 61.

The conductors 86 and 81 are cross-connected to opposite sides of the loop 31 at the junction of the comparatively wide upper portion 31a and a comparatively narrow lower portion 31h. Thus the output of the line 61 is connected across the portion 3112 of the complete loop 31, the impedance of this portion being a comparatively small percentage of the total loop impedance because of the narrowness of the slot forming the portion 31h.

It Will be evident that the coupling into the loop 31a, 3lb is due to the current ilow from the conductor 86 down one side of the loop portion 31h, up the other side of loop portion 3112 and into the conductor 8l as indicated by the arrows.

The loop 3l is in quarter wave length resonance. If properly excited and loaded, the potential on the capacitive end of the loop 31 will be symmetrical to ground. The slot forming 31h is made narrow so that the potential distribution on the loop 3l will remain practically symmetrical regardless of unbalance of the energy in the direct coupling lines 6l.

The termination of each of the coaxial lines formed by the center conductors 86 and 81 and by the associated outer conductors consisting of the walls of the holes 88 and 89, respectively, is inthe loop 31h. The electrical center of each coaxial line termination is not in the center of the lower end of the slot Ilb but is displaced by an amount approximately equal to the amount that the center conductor extends beyond the upper end of its outer conductor. Therefore, for maximum inherent symmetry the slot 31h should be made as narrow as mechanical limitations permit.

We claim as our invention:

1. A balanced detector comprising a tuned coaxial line havingr an outer conductor and an inner conductor, said inner conductor having an end section that is bifurcated and capacity terminated to form a tuned two-wire line section, a

pair of rectiers, one rectifier being connected between said outer conductor and one side of said bifurcated section, the other rectifier being connected between said outer conductor and the other side of said bifurcated section, means for applying signal to said coaxial line, said signal lying within the frequency range to which said coaxial line is tuned, means for simultaneously applying signal to said two-wire line section, said last signal lying within the frequency range to which said two-wire line section is tuned, and means for supplying from said diodes to a utilization circuit any beat frequency signal resulting from an instantaneous frequency difference of said signals.

2. A balanced detector comprising a tuned coaxial line having an outer conductor and an inner conductor, .said inner conductor having an end section that is bifurcated and capacity terminated to form a tuned two-wire line section, a pair of rectiers, one rectifier being connected between said outer conductor and one side of said bifurcated section, the other rectifier being connected between said outer conductor and the other side of said bifurcated section, means for applying signal to said coaxial line, said signal lying within the frequency range to which said coaxial line is tuned, means for simultaneously applying signal to said two-wire line section, said last signal lying within the frequency range to which said two-wire line section is tuned, and means for supplying from said diodes to a utilization circuit any beat frequency signal resulting from an instantaneous frequency diierence of said signals, said last means including a pair of conductors that pass through two longitudinal holes in said inner conductor and which are connected to an electrode of each of said diodes, respectively.

3. The invention according to claim 2 wherein said last means includes a transformer having two primary windings and a secondary winding andl wherein said pair of conductors are con.. nected to supply diode current to saidtwo primary windings, respectively.

4. A balanced detector comprising a tuned coaxial line having an outer conductor and an inner conductor, said inner conductor having an end section that is bifurcated and capacity terminated to form a tuned two-Wire line sectionl a pair of rectiers each having an anode and a cathode, one rectifier having its cathode connected through capacity to said outer conductor and having its anode connected through capacity to one side of said bifurcated section, the other rectier having its cathode connected through capacity to said outer conductor and having its anode connected through capacity to the other side of said bifurcated section, means for apply.. ing signal to said coaxial line, said signal lying within the frequency range to which said coaxial line is tuned, means simultaneously for applying signalto said two-wire line section, said last signal lying within the frequency range to which said two-wire line section is tuned, and means for supplying from said diodes to a utilization circuit any beat frequency signal resulting from an instantaneous frequency diierence of said signals, said last means including a pair of conductors that pass through two longitudinal holes in said inner conductor and which are connected to an electrode of each of said diodes, respectively.

5. The invention according to claim 4 wherein said last means includes a transformer having two primary windings connected in opposing relation and having a secondary Winding connected to said utilization circuit and wherein said pair of conductors are connected to supply diode current to said two primary windings, respectively.

6. A balanced detector comprising a tuned coaxial line having an outer conductor and an inner conductor, said inner conductor having an end section that is bifurcated and capacity terminated to form a tuned two-wire line section, a pair of .rectiliers each having an anode electrode and a cathode electrode, one rectifier having one of its electrodes connected through capacity to said outer conductor and having its other electrode connected through capacity to one side of said bifurcated section, the other rectifier having one of its electrodes connected through capacity to said outer conductor and having its other electrode connected through capacity to the other side of said bifurcated section, means for applying signal to said coaxial line, said signal lying within the frequency range to which said coaxial line is tuned, means comprising a loop adjacent to said two-wire line section for applying signal thereto, said last signal lying within the frequency range to which said two-wire line section is tuned, and means for supplying from Said diodes to a utilization circuit and beat frequency 5 signal resulting from an instantaneous frequency difference of said signals, said last means including a pair of conductors that pass through two longitudinal holes in said inner conductor and which are connected to an electrode of each of said diodes, respectively.

7. A balanced detector comprising a tunable coaxial line having an outer conductor and an inner conductor, said inner conductor having an end section that is bifurcated and capacity terminated' to forma tuned two-wire line section,

a pair of rectiers, one rectier being connected between saidl outer conductor and one side of said bifurcated section, the other rectifier being connected between said outer conductor and the other side of said bifurcated section, means for applying signal to said coaxial line, said signal lying within the frequency range to which said coaxial line is tuned, means for simultaneously applying signal to said two-wire line section, said last signal lying within the frequency range to which said two-wire line section is tuned, said last means comprising a pair of conductors that pass through two longitudinal holes in said inner conductor and which are cross-connected to opposite sides of said bifurcated section, and means for supplying from said diodes to a utilization circuit any beat frequency signal resulting from an instantaneous frequency difference of said signals.

8. A balanced detector comprising a tuned coaxial line having an outer conductor and an inner conductor, said inner conductor having an end section that is bifurcated and capacity terminated to form a tuned two-wire line section, a pair of rectiers each having an anode electrode and a cathode electrode, one rectifier having one of its electrodes connected through capacity to said outer conductor andhaving its other electrode connected through capacity to one side of said bifurcated section, the other rectiier having one of its electrodes connected through capacity to said outer conductor and having its other electrode connected through capacity to the other side of said bifurcated section, means for applying signal to said coaxial line, said signal lying within the frequency range to which said coaxial line is tuned, means for simultaneously applying signal to said two-wire line section, said last sig.. nal lying within the frequency range to which said two-wire line section is tuned, said last means comprising a pair of conductors that pass through two longitudinal holes in said inner conductor and which are cross-connected to opposite sides of said bifurcated section, and means for supplying from said diodes to a utilization circuit any beat frequency signal resultingfrom an instantaneous frequency difference of said signals, said last means including a second pair of conductors that pass through two longitudinal holes in said inner conductor and which are connected to an electrode of each of said diodes, respectively.

WILLIAM R. MERCER. BEN R. COLE. 

